Fabrication of molds and cores for metal casting was the application for which the inkjet-on-powder process for 3D printing was originally invented. Sachs, et. al. (U.S. Pat. No. 5,204,055) coined the term “Three Dimensional Printing” and taught the process as a method for fabricating metal casting molds and cores. The inventors specifically mention the use of organic resin adhesives in this patent (Col. 7 lines 34-44) for purposes of bonding ceramic powders, of which foundry sand is an obvious example. In the original embodiment, the adhesive that bonds together the particulate material is a component of the liquid binder that passes through the jets of the printing element (see col. 5, lines 39-50.) Later, Bredt (U.S. Pat. No. 5,851,465) taught a method and reactive materials system that included the printing of an adhesive-containing liquid binder (in this case colloidal silica) onto a ceramic powdered substrate containing a catalyst (in this case an acid) that caused the binder to solidify in a relatively short period of time, bonding the ceramic grains together by adhesion of silica gel, the product of coagulation of colloidal silica with an acid. Kasperchik et. al. (U.S. Pat. No. 6,742,456) taught a “crosslinking” reaction derived from acid-base cement chemistry.
Van der Geest (U.S. Pat. No. 6,403,002) disclosed a preferred sequence of operations for this style of 3D Printing: Applying a first material to a build bed, selectively applying a second material to the first material, curing the product, and repeating the steps. A soluble, solid adhesive was combined with an inert filler such as sand to form the first material, and the second material included a solvent (e.g., water and a flow promoter or “carrier”). Van der Geest's motivation for removing the adhesive component from the printed fluid was to enhance the reliability of the printing process (col. 1, lines 21-26) when a “conventional inkjet printer” is adapted for 3D Printing.
The commercialized versions of the process embodied by Sachs et. al. have long been divided into two camps. One, using repurposed desktop printing hardware, may be represented by van der Geest and various early products marketed by Z Corporation, such as the ZPrinter 510. In these machines, printheads were considered to be consumable, and only cursory effort was made to maintain cleanliness of the printing elements. In the other camp, industrial printing equipment was developed using more expensive but more reliable printing hardware. Products manufactured by VoxelJet and EXOne are examples of these. In this second camp, reactive binders and monomers are quite feasible to print through the industrial inkjet printheads, as long as sufficient effort is made to keep the aggressive reagents from corroding or solidifying the printing elements. This adds to the cost of the machine, but makes it much more adaptable to the industrial environment.
Brodkin et al. (U.S. Pat. No. 6,322,728) disclose methods for binding ceramic powders (including silica, alumina, mullite, spinel and zirconia) by printing a reactive monomer (acrylic and methacrylic acids) containing a free-radical initiator that may be photo-sensitive or heat-sensitive. Williams discloses a further technique in which a free-radical initiator is mixed with the particulate substrate and an acrylic monomer is printed from an inkjet printhead to form a three dimensional article.
Davidson et al. (U.S. Pat. No. 7,037,382) disclose methods for recirculating the granular medium within a 3D Printer, allowing the granular material to be reused from one build to the next. The advantages to recycling the unused materials in a 3D printing process are quite attractive. Besides saving on the cost of new materials, one reduces the handling and disposal of a toxic waste stream emerging from a 3D printer that adds to the cost of operation.